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Polyurethane foam also fills the crack or joint with a resin that expands in the presence of moisture. Unlike epoxy, polyurethane foam is flexible enough to accommodate movement in the crack or joint due to changing soil pressures or minor settlement.

Since polyurethane resin systems expand during the injection, less resin is required to accomplish the repair and can be much more economical to install—especially in wide cracks and loose soil conditions.
Wet or actively leaking cracks and joints often will have better results being injected with polyurethane foam. In fact, because polyurethane resin systems are moisture reactive, they may actually require the crack to be pre-wet with a small amount of water in order to activate the full expansion of the resin.
There are epoxies that will work well in wet conditions as the epoxy will displace the water during the injection process, but extra care needs to be taken to “flush out” any resin that combines with the water. (More on how this is accomplished later.)